1. Field of the Invention
The present invention relates to the use of spectroscopic methods to detect and measure very low levels of contaminants in water, oils, and other media.
2. Background of the Invention
Contamination of water, including drinking water, ground water, and seawater, poses considerable problems for human health and for the environment. Certain industrial processes also require water that is free from organic materials at the level of parts per million (ppm) or parts per billion (ppb). For different reasons, it can be important to detect small quantities of water in organic matrices such as crude oil, kerosene, lubricating oil, or gasoline.
A range of analytical procedures for the determination of hydrocarbons in water is available in the literature. A number of ISO standards have been published that specify methods for measuring organic contaminants in water (see Table 1).
TABLE 1Examples of ISO standards for organic contaminant analysis in waterStandard NumberTargetMethodISO 10301: 1997Highly volatile halogenatedGas chromatographic (GC)hydrocarbonsmethodsISO 10695: 2000Selected organic nitrogen andGC methodsphosphorus compoundsISO 11423-2: 1997Benzene and some derivativesExtraction and GCISO 15680: 2003A number of monocyclic aromaticGC determination using purge-hydrocarbons, naphthalene andand-trap and thermal desorptionseveral chlorinated compoundsISO 17495: 2001Selected nitrophenolsSolid-phase extraction and gaschromatography with massspectrometric detection(GC/MS)ISO 17858: 2007Polychlorinated biphenylsGC/MSISO 17993: 200215 polycyclic aromatic hydrocarbonsHigh-Performance Liquid(PAH)Chromatography (HPLC) withfluorescence detection afterliquid-liquid extractionISO 18857-1: 2005Selected alkylphenolsMethod for non-filtered samplesusing liquid-liquid extraction andGC/MSISO 6468: 1996Certain organochlorine insecticides,GC method after liquid-liquidpolychlorinated biphenyls andextractionchlorobenzenesISO 7981-2: 2005PAHHPLC with fluorescencedetection after liquid-liquidextractionISO 8165-1: 1992Selected monovalent phenolsGC method after enrichment byextractionISO 8165-2: 1999Selected monovalent phenolsDerivatization and GCISO 9377-2: 2000Determination of hydrocarbon oilSolvent extraction and GCindex
All of the listed ISO methods include at least an extraction/concentration step, or a purge-and-trap step; some include a derivatization step. All are essentially laboratory-based methods, unsuitable for implementation in the field, or for continuous monitoring of process or waste fluids.
The determination of water content in hydrocarbon streams such as crude oil, fuel oil, or mineral oil is also of considerable economic importance. The presence of even quite small amounts of water can interfere with the processing of crude oil, and can compromise the performance of fuel oil. Water in machine or engine oil can lead to accelerated wear or corrosion of machine or engine parts. Consequently, a range of standard methods exist for the determination of water content in oil (see Table 2).
TABLE 2Examples of standards for water determination in oilStandard NumberTargetMethodASTM D4006 - 07Standard Test Method for WaterDistillation(ISO 9029)in Crude Oil by DistillationASTM D95Test Method for Water inDistillationPetroleum Products andBituminous Materials byDistillationASTM D96Test Methods for Water andCentrifugationSediment in Crude Oil byCentrifuge Method (FieldProcedure)
In general, the standard methods are based on distillation, which is a complex method and not suitable for field or process use, or on centrifugation.
A Fourier-transform infrared (FTIR) spectroscopic method for analyzing organics in water has the potential to be a fast, non-complex technique, offering the elimination of the extraction/concentration step, the purge-and-trap step, or the derivatization step. However, existing procedures for IR analysis of organics in water generally include an extraction step (see, for example Application Note: Determination of Oil and Grease in Water with a Mid-Infrared Spectrometer, PerkinElmer Inc., 2009). While a number of small, portable instruments are available that lend themselves to field use, they too require an extraction step (see, for example Application Note: New ASTM Test Method Offers Quick and Easy Oil and Grease, Sandra Rintoul, Wilks Enterprise Inc.) The extraction step is carried out because (a) water absorbs very strongly in the mid-IR, making transmission measurements impossible and (b) the ATR method has not generally been considered sensitive enough to detect parts per million of organics in water.
FIG. 1 illustrates the mechanism of the attenuated total reflectance (ATR) effect in an infrared element (IRE) such as a zinc selenide, zinc sulfide, or diamond crystal. When the IRE is immersed in a fluid sample, the evanescent wave penetrates into the sample to a depth determined by the ATR equation:
      d    p    =      λ          2      ⁢      π      ⁢                          ⁢                                    n            p                    ⁡                      (                                                            sin                  2                                ⁢                θ                            -                              n                sp                2                                      )                                    1          ⁢                      /                    ⁢          2                                    where dp is the penetration depth at each bounce                    λ is the wavelength of the radiation            np is the refractive index of the crystal            θ is the angle of incidence of the light beam            nsp is the ratio of the refractive indices of sample and crystal                        
Based on this equation, the signal penetration depth into water, using any of the conventional ATR materials, is in the range of a few micrometers. While this enables the use of mid-IR spectroscopy in high-absorbing media such as water by decreasing the path length dramatically versus transmission methods, it limits the sensitivity of the technique. Typically, an ATR measurement is expected to be at least an order of magnitude less sensitive than a comparable transmission measurement, even at the lowest obtainable transmission path lengths. Considerable effort has been made to develop a method that combines the desirable attributes of ATR-FTIR methods (rapid, direct, reproducible measurement, effective in water) with the sensitivity of extraction methods or transmission-FTIR.
Efforts to eliminate a separate extraction step, or other concentration step, have included the use of so-called molecular enrichment layers, or solid-phase micro-extraction layers (SPME layers), on ATR crystals. These layers are applied to the surface of the ATR element so as to selectively concentrate the target analyte close to the crystal surface and within the region that is sampled by the evanescent wave during the ATR measurement. Much of the published work in this area has involved thin polymeric coatings on zinc selenide or zinc sulfide ATR crystals (see, for example, “Amplified” Fiber-Optic ATR Probes with Improved Detection Limits in the Mid IR, presented at the Federation of Analytical Chemistry and Spectroscopy Societies (FACSS) Meeting in Providence, R.I., 2002, by P. Melling, M. Thomson, B. Mizaikoff, and M. Karlowatz; From the Lab to the Field—Recent Developments in Polymer Coated ATR, thesis by M. Karlowatz, Georgia Institute of Technology, 2004; Development of an SPME/ATR-IR chemical sensor for detection of phenol type compounds in aqueous solutions, J. Yang and M-L Cheng, The Analyst, 2001, 126, 881-886). These layers, typically comprising hydrophobic polymers such as ethylene-propylene copolymer or PTFE, have the effect of selectively concentrating hydrophobic analytes close to the ATR surface and thus enhancing the resulting FTIR signal. While this method has proven to be very sensitive, it is difficult to calibrate and the robustness of the coated ATR elements has not been demonstrated. Coatings made from oxide nanoparticles, from zeolites, and sol-gel silica have also been used as SPME layers to promote ATR detection of organics in water.
A related, but different, approach involves combining an ATR crystal with a membrane-based micro liquid-liquid extraction cell to create a sensor which performs a micro-extraction of organics from an aqueous stream to deliver a concentrated sample to the ATR element (M. Vacarcel et al., ATR-FTIR membrane-based sensor for the simultaneous determination of surfactant and oil total indices in industrial degreasing baths, Analyst, 2006, 131, pp. 415-421). Like the SPME layers, this method has the potential to be very sensitive. However, it is essentially a laboratory-based approach which would be difficult to implement in the field. Furthermore, as described by Valcarcel, it involves the use of environmentally unacceptable solvents such as carbon tetrachloride.
Water content in oils, e.g. in industrial lubricants, is also an important measurement, and presents a similar challenge. Many currently accepted standard methods, such as Karl Fischer Titration, for example, are laboratory-based. An FTIR method is described in ASTM E2412-10 Standard Practice for Condition Monitoring of Used Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry, and an improved method using FTIR has been proposed recently (US Patent Application 2009/0257047 by Higgins and Seelenbinder, and Application Note 101: On-Site, Low Level Quantitative FTIR Analysis of Water in Oil Using a Novel Water Stabilization Technique, by Higgins and Seelenbinder, A2 Technologies). The method of Higgins and Seelenbinder involved treating the oil sample with a surfactant to stabilize and evenly distribute the water content before measuring a transmission spectrum and using a suitable univariate or multivariate calibration to determine the water content.
All of the FTIR methods described for both oil-in-water and water-in-oil analysis involve specific steps to concentrate the analyte, or stabilize its distribution throughout the sample, before carrying out the FTIR analysis. In the ATR measurement, which is the only practical method for measuring FTIR spectra in aqueous samples due to the very high IR absorption of water, the development of a suitable process to concentrate the analyte has been extensively studied. The concentration is Carried out either as a separate step, resulting in a sample where the analyte is concentrated, or by treating a conventional ATR material, such as zinc selenide, zinc sulfide, or diamond, with specialized coatings which enhance the analyte concentration near the ATR surface, so as to facilitate analysis down to the level of parts per million (ppm).
In light of the extensive literature describing pre-concentration or stabilization of the analyte, and pretreatment of ATR crystals to enhance analyte detection by surface concentration, the present discovery that certain ATR materials are sufficiently hydrophobic to promote analyte enrichment close to the ATR surface, and thus to enable detection of organics in water down to ppm levels, is completely contrary to expectations. Similarly, the ability of more hydrophilic ATR materials to enable detection of ppm water in organic media such as oil is surprising.